This invention generally relates to an improved glow plug engine, and is specifically concerned with an improved model airplane engine of the type having a propeller on an output shaft and a needle valve for controlling the fuel-to-air mixture entering the carburetor, wherein the improvement comprises the use of a pinch valve on the fuel line in lieu of the needle valve for regulating the fuel-to-air mixture entering the carburetor which is spaced away from the propeller a greater distance than the needle valve.
Needle valves for regulating the fuel-to-air mixture entering the carburetor of a glow-plug engine, such as a model airplane engine, are well known in the prior art. Such needle valves generally comprise a shaft having a very fine screw thread around its exterior surface, and a tapered, needle-like member at its distal end which is reciprocally movable across a fuel path defined within the body of the engine. The fine screw thread associated with such needle valves allows a precision adjustment to be made as to the size of the fuel opening in the engine. Such needle valves further include either a spring rachet or an adjustable drag nut to keep the valve from backing out as a result of the considerable vibration they are subjected to during the operation of the engine.
While such needle valves are capable of performing their intended purpose of precisely achieving and maintaining a desired fuel-to-air mixture, the applicant has observed a number of shortcomings associated with such valves. One major shortcoming involves cost. While the operation of a needle valve is a simple enough concept, such valves require the manufacture and assembly of a number of precision-made components, including the fine screw thread used to adjust the position of the valve, and the previously mentioned spring rachet or adjustable guard nut assemblies to secure the screw shaft of the valve in the desired location after adjustment. A second shortcoming associated with the use of such valves in glow-plug engines is that the location of the needle valve is often unsafe and inconvenient when the glow-plug engine is mounted in a model car, boat or airplane. Often the engine housings used in such models must be modified to provide access to the needle valve of the engine.
Other major shortcomings arise specifically in the context of model airplane engines having an output shaft which terminates in a propeller. The needle valves used in these engines are usually located quite close to the rotating propellers of these engines, forward of the cylinder. Such close distancing has resulted in injuries ranging from cut to severely injured fingers, damaged tendons, and major loss of blood. Moreover, both the frequency and severity of these injuries appears to be increasing as a result of two factors. First, while in the past such model aircraft engines typically had a displacement between 0.30 and 0.60 cubic inches, the average size of such engines is getting larger. Presently, there are a number of model airplane engines of 1-2 cubic inches in displacement, which generates several horsepower. Secondly, the use of carbon-fiber reinforced plastic propellers has exacerbated the injuries caused by such engines, since such propellers are relatively sharper edged than the wooden propellers which were used more frequently in the past. Additionally, such carbon-reinforced plastic propellers will not break as easily as wooden propellers when they strike an object, which increases the likelihood that the propeller will break a finger or other body part placed in its way before it will itself be broken.
Another shortcoming associated with needle valves used in model airplane engines is the lack of adjustability of the fuel-to-air mixture regulated by the needle valve once the model airplane is in flight. Optimizing the fuel-to-air mixture in most model engines is often a delicate and difficult task, requiring a considerable amount of time with trial-and-error adjustment of the needle valve. The optimality of this setting changes from day to day, or even in the same day, as atmospheric conditions change. Consequently, many modelers, in their efforts to obtain maximum performance, spend considerable time adjusting the needle valve prior to each flight. The lack of any means to re-set the needle valve after the model airplane is in flight often results in the flight being executed with a less than optimum fuel-to-air ratio, which can cause engine damage or cause the model's engine to stop running, which sometimes results in a crash. In competitions, the lack of any way to readjust a poorly adjusted valve results in a loss of points which can cost the modeler the contest. Additionally, the competitors are often tempted to continue a flight with a poorly adjusted valve when they could have landed the model safely before the engine quit entirely. Such situations have resulted in the destruction of many valuable models.
Although there has been some recognition of the safety shortcoming, and although there have been solutions to this problem proposed and attempted, there has never been a satisfactory solution that would overcome all of these shortcomings. While there have been various devices proposed and tried for keeping the needle valve in its usual position and putting the adjustment means at some distance, such as the use of a flexible cable, gears, pulleys, etc., these devices all have major drawbacks such as cost, complexity, bulkiness, etc., and often do not allow for the precise adjustment necessary due to the "wind-up" characteristics associated with such drive trains. While it is possible to mount a needle valve separately from the carburetor by means of a tube disposed between the valve and the carburetor, this feature is available commercially only for relatively few engine models, and is expensive because a bracket and additional fuel line attachment means are necessary in addition to the needle valve, and because the needle valve must duplicate portions of the part of the carburetor which usually forms the female portion of the valve. It is also bulky and offers little flexibility in placement. Finally, there are commercially available needle valves which are capable of remote operation in combination with a servomechanism which can be added to some engines. However, they are quite expensive and are only applicable for a few specific engine models. Also, relatively few modelers have added this feature to their airplanes, because of the added installation complexity, the added expense, weight and space requirements of an additional servomechanism, and the fact that many radio systems, especially those used by beginners, are not equipped with the additional adjustable channel required. Further, such servo-controlled needle valves cannot be used with model airplanes which are not radio controlled.
Clearly, there is a need for an improved model airplane engine having a valve for controlling the flow of fuel into the carburetor of the engine which can be adjusted at a location which is at least more remote from the rotating propeller than the location where the needle valves are typically located, and which could also be adjusted after the model airplane is in flight so that fuel-to-air optimality could be easily maintained at all times. It would further be desirable if the valve associated with such an improved engine could be easily and precision-adjusted without the need for any flexible cables or expensive gear trains and without any intervening "wind-up" problems. Ideally, such a valve could be mounted at a variety of locations with respect to the engine so that the valve could be easily accessed and operated no matter what shape or form the engine mounting or cowls took. Finally, it would be highly desirable if the valve used in conjunction with such an engine could be easily retrofitted on any prior art model airplane engine with a minimum amount of mounting hardware and effort, and was inexpensive to manufacture and to install, and provided a highly accurate and reliable fuel flow adjustment that was easy to operate.